1. Field of the Invention
The invention relates generally to measuring systems on traction vehicles, such as locomotives or transit vehicles, and more particularly, the invention relates to systems and methods for estimating the speed of the vehicle""s wheels without using speed sensors.
2. Brief Description of the Prior Art
Locomotives and transit vehicles as well as other large traction vehicles are commonly powered by electric traction motors coupled in driving relationship to one or more axles of the vehicle. Locomotives and transit vehicles generally have at least four axle-wheel sets per vehicle, with each axle-wheel set being connected via suitable gearing to the shaft of a separate electric motor commonly referred to as a traction motor. In the motoring mode of operation, the traction motors are supplied with electric current from a controllable source of electric power (e.g., an engine-driven traction alternator) and apply torque to the vehicle wheels which exert tangential force or tractive effort on the surface on which the vehicle is traveling (e.g., the parallel steel rails of a railroad track), thereby propelling the vehicle in a desired direction along the right of way. Alternatively, in an electrical braking mode of operation, the motors serve as axle-driven electrical generators such that torque is applied to their shafts by their respectively associated axle-wheel sets, which then exert braking effort on the surface, thereby retarding or slowing the vehicle""s progress. The motor generated energy can be dispersed in a grid of resistors.
The need to measure the rotational speed of the axle-wheel sets has been recognized in the art. In some locomotives, the locomotive speed or tangential wheel speed is calculated from measured motor rotor revolutions per minute (xe2x80x9cRPMxe2x80x9d) values given the diameter of the associated wheel. Conventionally, a speed sensor or revolution counter is coupled to sense the rotational speed of an output shaft of each drive motor. These RPM signals are converted to wheel rotational speed from the known gear ratio of the mechanical coupling between the motor shaft and wheel axle. Wheel rotational speed is converted to vehicle linear speed based upon the assumed diameter of each driven wheel. Additionally, the speed sensor data may be used to detect a locked axle condition. If a locked axle condition occurs on a given axle while the locomotive is moving, the rotational speed of that axle decreases to zero, and all other axles rotate at a speed corresponding to the speed of the locomotive. Thus, a locked axle condition can be detected whenever a substantial difference in speed is sensed in one axle relative to the other axles.
However, some locomotives are not equipped with speed sensors capable of measuring the revolutional speed of the output shaft of the drive motors. Additionally, speed sensors operate in a treacherous environment and have a high failure rate. Accordingly, it would be advantageous to provide a wheel speed estimation system that does not require a speed sensor to be mounted on the output shaft of each drive motor capable of determining the vehicle""s linear speed and capable of identifying a locked axle condition.
In one embodiment, the invention is a method of estimating the wheel speed of a vehicle propelled by DC motors mechanically coupled to at least one wheel on the vehicle so as to rotate said one or more wheel. The method includes measuring the armature current and the field current in each of the DC motors and measuring a voltage applied to each of the DC motors. The method further includes determining a rotational speed as a function of the measured currents and voltage, and applying a correction factor to the determined rotational speed to obtain a corrected rotational speed. In one embodiment, the method further includes converting the corrected rotational speed into a linear wheel speed.
In another embodiment, the invention is a system for estimating the wheel speed of a vehicle propelled by DC motors mechanically coupled to at least one wheel on the vehicle so as to rotate said one or more wheel. The system includes at least one current detector configured to measure armature and field current in each of the DC motors and produce a signal proportional to said currents and a voltage detector configured to measure voltage applied to each of the DC motors and produce a signal proportional to said voltage. The system further includes a processor configured to determine a rotational speed as a function of the measured currents and voltages and configured to apply a correction factor to the determined rotational speed to obtain a corrected rotational speed. In one embodiment, the processor includes a motoring speed estimator module configured to determine a rotational speed using the current and voltage signals when the vehicle is operating in a motoring mode and produce a motoring rotational speed signal proportional to said rotational speed. The processor further includes a braking speed estimator module configured to determine a rotational speed using the current and voltage signals when the vehicle is operating in a braking mode and produce a braking rotational speed signal proportional to said rotational speed. The processor further includes a speed correction module configured to receive said motoring and braking rotational speed signals and at least one reference signal, said speed correction module configured to apply a correction factor to said rotational speed signals and translate said rotational speed signals to linear speed signals.
In another embodiment, the invention is a method of detecting the occurrence of a locked axle condition in a vehicle propelled by multiple DC traction motors. The method includes measuring armature currents for each of a plurality of DC traction motors connected to a plurality of axles. The method further includes determining a minimum armature current from the measured currents, determining a maximum armature current from the measured currents, and comparing the minimum armature current to the maximum armature current to identify the occurrence of a locked axle condition. In one embodiment, the method further includes determining if the locomotive is operating in a motoring mode or a braking mode, and adding a value to the minimum armature current proportional to an Emf value for the traction motor multiplied by the rotational speed of the traction motor and divided by a resistance value for the traction motor when the vehicle is operating in the motoring mode. In one embodiment the method further includes estimating the rotational speed of the traction motor by measuring the armature current and the field current in a DC motor, said DC motor being mechanically coupled to at least one wheel on the vehicle so as to rotate said one or more. wheel, measuring a voltage applied to the DC motor, and determining a rotational speed from the measured currents and voltage.